Simplify. Rewrite the expression in the form $z^n$. $z^{-11}\cdot z^{-15}=$
Recall that $x^n\cdot x^m=x^{n+m}$. $\begin{aligned} z^{-11}\cdot z^{-15}&=z^{-11+(-15)} \\\\ &= z^{-11-15} \\\\ &= z^{-26} \end{aligned}$